EVERY ONCE IN a while in medicine a new discovery comes along that, on its surface, seems to make no sense at all.

What if I were to tell you that a popular drug — one that's been sampled by two out of every five Americans and is renowned for its relaxation and anti-nausea properties — could actually cause severe bouts of nausea and vomiting among long-term users? These symptoms are worse in the morning and seem to be most effectively relieved by a long hot shower. Hmmm, smells a bit skunky, right?

But, recently published reports seem to confirm the phenomenon. And, you might be more than a bit surprised by the culprit: THC.

Cannabinoid hyperemesis (THC-associated vomiting) was first described in 2004 but remains a relatively unknown and underappreciated malady. A recent case series by Douglas A. Simonetto and colleagues at the Mayo Clinic in Rochester, Minn., compiled data on 98 patients with this condition — and in the process they've put it on the map as a medical diagnosis.

The accumulating evidence about cannabinoid hyperemesis gives us this typical patient profile: a chronic cannabis user younger than 50 who develops periodic and severe abdominal pain associated with nausea and vomiting that tends to be worse in the morning and to improve with hot showers or baths.

For example, in Simonetto's report (published in February 2012 in Mayo Clinic Proceedings), the study team reviewed 1,571 charts of patients seen at their institution between 2005 and 2010 and identified 98 that met their criteria for cannabinoid hyperemesis (long-term cannabis users with recurrent vomiting not explained by any other medical illness). Of these, each one was younger than 50 and the vast majority (95 percent of those for whom data was available) used cannabis at least twice a week. Of the 57 patients for whom the effect of hot water immersion was documented, 51 (91 percent) reported relief. Most patients (86 percent) also reported abdominal pain.

From these results, the authors have proposed specific criteria for the diagnosis of cannabinoid hyperemesis — including most of the above characteristics. As to the cause of the paradoxical vomiting because of cannabinoids, several theories have been proposed, including the inhibitory effects of cannabis on peristalsis (the wavelike impulses that propel food through the intestines). It may also be that chronic use changes the effects of cannabinoids on THC receptors in the brain.

More investigation is needed, however, and as of now we can only speculate as to why hot showers are such an effective treatment (it could be because hot water draws blood supply away from the gut and to the skin). Despite the uncertainty, this new evidence does have a couple important implications for how we think about cannabinoids:

 Today's cannabis is not the same as your father's puff the magic dragon. In particular, synthetic and oral cannabinoids can lead to much higher and prolonged exposure to THC or similar compounds — and this would, presumably, increase the risk of cannabinoid hyperemesis syndrome. In particular, "herbal incense" products like Spice and K2 can cause effects — including prolonged psychosis — that were rarely observed from marijuana alone in the era of dirt grass and flower power. While the evidence is accumulating that cannabinoids may be helpful in the treatment of a number of uncomfortable medical conditions — including those that cause nausea and vomiting — one must be careful with the amount and delivery of the drug. Like any drug, cannabis is not free of risk or side effects.

 If you are a long-term cannabis user suffering from severe bouts of nausea and vomiting, there may be simple treatments. First, try a nice, long, hot shower. The second (and medically advised) treatment for cannabinoid hyperemesis is to stop using cannabis. In Simonetto's case series, of those reporting have quit cannabis, six out of seven (86 percent) had complete resolution of symptoms.

In medicine, like in life, it is always good to constantly question and challenge assumptions. The long-standing assumption about cannabis is that its main danger is as a "gateway" drug — and that it is otherwise quite safe and side effect free. We are starting to learn that this is not exactly true.

Dr. Dustin W. Ballard is an emergency physician at Kaiser Permanente San Rafael and the author of "The Bullet's Yaw: Reflections on Violence, Healing and an Unforgettable Stranger." His Medically Clear column appears every third Monday; follow him on Twitter at http://twitter.com/dballard30.